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At our greenhouse, in my teen years, I was the chef de cuisine of potting soils. My job was to blend wholesome ingredients in just the right proportions to create a mix that plant roots would love. It also involved heavy lifting and inhaling in a lot of dust particles, so it's a job and title I don't miss.

At that time, our standard potting soil mixture included coarse-fibred peat moss, a white "popped" rock called perlite and topsoil.

The first step was to heat the topsoil in an electric pasteurizer that looked like a big oven with thin metal hot plates criss-crossing the interior for even heating.

The soil was heated just enough to kill all of the weed seeds, diseases and insects, yet not overheated to the point where it would release compounds toxic to plant roots.

The next step was to mix the steaming topsoil with the correct portions of peat moss and perlite (to loosen the potting soil) and a bit of limestone tossed in to sweeten the recipe (to counteract the peat moss acidity).

The combination we used was common in the greenhouse industry for years, but today it's virtually disappeared.

Although it sounds counterintuitive, it was the soil component of the potting mixture that was the problem. Topsoil has a lot of virtues - it anchors roots well, holds water and nutrients, and is plentiful. But when it's incorporated into a potting soil mixture, its weaknesses become apparent. I'll explain.

To start with, although topsoil (or loam, as it's often called) is thought of as a homogeneous material, its composition is remarkably variable. There are clay loams, sandy loams, silty loams, silty-clay loams . well, you get the picture. It's because of that variability that it can be difficult to create a consistent potting soil mix that contains topsoil.

Topsoil is also very heavy, which isn't such a big deal in small pots, but is a pain (literally) in big pots. Moving a large deck pot that contains a fair bit of topsoil requires herculean strength to move even a few feet. And, let's face it, it's tough to enjoy your flowers when you're wearing a back brace.

Another troubling side of topsoil is its chemistry. Even trace amounts of certain chemicals can spoil a potting soil recipe. I vividly recall one year when the leaves of our tomato seedlings inexplicably began to twist and then, shortly thereafter, the tomato plants died. No, my mixing skills had nothing to do with it. The problem was in fact attributed to trace amounts of weed killer that somehow made it into our stockpile of topsoil.

Not a good day, but the lesson that experience taught us was to eliminate variables as much as possible when it comes to potting soil mixes. Needless to say, it was the last time topsoil was incorporated into our mixtures. Today, we use a professionally formulated mix to grow our greenhouse plants. It's also the same one we sell. It's an excellent potting soil: Lightweight, consistent and, from my perspective, worry free.

That doesn't mean you can't grow excellent plants in potting mixtures that include topsoil. Just be aware of topsoil's variances and extra weight. If you love to tinker and have an overwhelming desire to be a potting soil chef de cuisine, then by all means create your own special recipe.

Thankfully, messing up a potting soil recipe for tomato plants is far easier to live down than say forgetting Valentine's Day. Three years in a row. Not that I'd know anything about that .

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